In general, a hydraulic system is mounted in a construction machine to operate various operating devices. The hydraulic system receives power from an engine and operates a hydraulic pump, and operates various operating devices by working oil discharged from the hydraulic pump.
An electronically controllable electronic hydraulic pump is known as the hydraulic pump. Further, the hydraulic pump includes a hydraulic pump in a pressure control type.
The pressure control type electronic hydraulic pump may control a size of finally output pump torque by electronically controlling an angle of a swash plate. Further, the pressure control type electronic hydraulic pump is a type that controls pressure of the pump in proportion to a detected pressure value of working oil.
Hereinafter, the “pressure control type electronic hydraulic pump” is abbreviated as a “pump”.
As the related art, Patent Literature 1, “Apparatus and Method for Controlling Hydraulic Pump for Construction Machine”, which was filed by the applicant of the present disclosure and published is known.
Patent Document 1 relates to a method of controlling output torque of a hydraulic pump, and is a technology of mapping torque response performance of an engine with a time constant corresponding to a pump torque control means based on an engine speed.
In order to find a time constant used for the control in Patent Literature 1, it is very important to recognize a dynamic characteristic according to an engine speed, and the hydraulic system in the related art controls output torque of a hydraulic pump by setting a time constant based on a reach of a load pattern from a standby load (zero or a predetermined level) to a full load.
In the time constant control method, when a load is not the full load, a change rate of output torque of a hydraulic pump is decreased, so that an engine speed is not decreased, but an operation speed is unintentionally decreased, thereby degrading workability.
On the other hand, performance of the engine is degraded according to an increase in the operation time thereof, so that the dynamic characteristic of the engine is changed. Accordingly, there is a problem in that even though the pump is controlled by loading a torque change rate map for each load, to which a dynamic characteristic of the engine is reflected, when a dynamic characteristic of the engine is changed, the already loaded torque change rate map for each load cannot smoothly control the hydraulic pump.